Learning management systems have become a powerful tool to support and manage online learning in higher education. However, in Vietnam, English as a Foreign Language (EFL) learners are used to direct face-to-face teaching, making learners' acceptance of technology-mediated online learning of English an issue of concern. In this study, we investigated the factors affecting students' acceptance of an elearning system called Cambridge Learning Management System (CLMS) that was implemented at a Vietnamese university. A quantitative survey, based on the baseline Technology Acceptance Model extended with three external factors from the Usability Evaluation Framework, was administered to 256 participants who had taken English courses on the CLMS. Results from factor analysis and structural equation modelling revealed that Perceived Usefulness and Ease of Use were not correlated but mediated between learner intentional behavior and the external factors of Content and Resources, Instructional Assessment, and System Accessibility. The study offers implications for future practice and research regarding the adoption of elearning systems for blended English learning.